1. Field of the Invention
The present invention relates to the manufacture of circuit boards and integrated circuit packages. More particularly, the invention relates to low storage modulus, electrically conductive thermal interfaces for integrated circuit packages.
2. Description of the Related Art
Integrated circuits are well known industrial products, and are used for a wide variety of commercial and consumer electronic applications. They are particularly useful in large scale applications such as in industrial control equipment, as well as in small scale devices such as telephones, radios, and personal computers.
As the desire for more intensive electronic applications increases, so does the demand for electrical systems that operate at faster speeds, occupy less space, and provide more functionality. To meet these demands, manufacturers design electrical and electronic devices containing numerous electrical components residing in relatively close proximity. These components tend to generate large amounts of heat which must be dissipated by some means to avoid failure or malfunction of the device.
Traditionally, electronic components have been cooled using forced or convective circulation of air within the housing of the device. Cooling fans are often provided as an integral part of an electronic device or are separately attached thereto for increasing the surface area of the integrated circuit package which is exposed to air currents. Such fans are employed to increase the volume of air which is circulated within a device's housing. U.S. Pat. No. 5,522,700 teaches the use of a typical fan device for dissipating heat from an electronic component. U.S. Pat. No. 5,166,775 describes an air manifold mounted adjacent to an integrated circuit for directing air jets onto electronic devices mounted to the circuit. The air manifold has an air inlet and a plurality of outlet nozzles positioned along the channel for directing air onto the electronic devices.
Unfortunately, as integrated circuits continue to decrease in size while power densities increase, simple air circulation is often insufficient to adequately cool circuit components. Heat dissipation beyond that which is attainable by air circulation can be achieved by attaching a heat sink or other thermal dissipation device to the electronic component. U.S. Pat. No. 4,620,216 describes a unitary heat sink for a semiconductor package having a plurality of cooling fin elements, which heat sink is used to cool high density integrated circuit modules. U.S. Pat. No. 5,535,094 teaches the combined use of an air blower and a heat sink. It teaches a module which has an integral blower that cools an integrated circuit package. The blower is attached to a heat sink that is mounted to the integrated circuit package. Heat generated by the integrated circuit conducts to the heat sink. The blower generates a stream of air that flows across the heat sink and removes heat from the package.
It is known in the art to use a thermal or electrical interface to attach such thermal dissipation devices to a heat emitting component. However, conventional interfaces have been known to suffer from several disadvantages. Interfaces used in the semiconductor industry typically comprise metal interfaces or polymer adhesives filled with conductive fillers. Metal interfaces such as solder, silver, and gold provide low resistivity, but have a high storage modulus and are not suitable for large IC dice. Furthermore, polymer adhesives can be very low modulus, but their resistivity is too high. As the amount of heat emitted from chips gets higher, there is a need for a thermal interface for use with an integrated circuit package or semiconductor die, which thermal interface has a low modulus as well as high thermal and electrical conductivity. There is also a need for such interfaces to be capable of being assembled and processed at low temperatures, such as about 200° C. or less.
The present invention provides a solution to this problem. According to the invention, a porous, flexible, resilient heat transfer material is formed, which material comprises network of metal flakes. Such heat transfer materials are preferably produced by first forming a conductive paste comprising a volatile organic solvent and conductive metal flakes. The conductive paste is heated to a temperature below the melting point of the metal flakes, thereby evaporating the solvent and sintering the flakes only at their edges. The edges of the flakes are fused to the edges of adjacent flakes such that open pores are defined between at least some of the adjacent flakes, thereby forming a network of metal flakes. This network structure allows the heat transfer material to have a low storage modulus of less than about 10 GPa, while having good electrical resistance properties.